What does a horizontal line represent on a distance-time graph?

It represent the x axis the independent variable

t represent the x axis the independent variable

Explanation:

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when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.